1. Field of the Invention
The invention relates generally to a photovoltaic device formed by the deposition of semiconductor based layers on a substrate. More specifically, the invention relates to the deposition and recrystallization of silicon to form solar devices on a non-semiconductor substrate, discrete platens or continuous sheet.
2. Description of Related Art
Prior art in this area includes U.S.2010/0059107; U.S. 2008/0295882; U.S.2008/0072953; U.S.2008/0023070. Preceding references incorporated in their entirety herein by reference. None of the cited prior art effectively addresses the primary issue for solar cells, namely low manufacturing cost coupled with commercial level conversion efficiency; solar cell module costs must be below $0.50/watt to begin to achieve parity with conventional utility pricing.
Plasma spraying has been discussed for many years for forming silicon semiconducting devices including silicon solar cells. Such efforts have not found ready commercialization because of the low quality of the sprayed silicon. Interest in plasma sprayed semiconducting silicon has been rekindled recently in the hope of providing a low cost manufacturing method for silicon solar cells. Zehavi et al. have reported recent efforts in plasma spraying high-quality silicon in U.S. 2008/0220558.
Tamura, “Fabrication of poly-crystalline silicon films using plasma spray method”; Solar Energy Materials and Solar Cells, 34 (1994) 263-270, investigated solar cells on graphite substrates, particularly glassy graphite and woven graphite. While feasibility was demonstrated, serious obstacles prevented commercialization of the disclosed processes. Among the obstacles disclosed were the availability of pure, or even ultra-pure, silicon powder, contamination of the deposited Si layer from the plasma spray gun, efficient utilisation of the Si powder from the deposition perspective. From the substrate perspective the primary problems were adherence to the graphite, graphite purity, and graphite integrity which results in the graphite peeling away in layers from itself, and the high cost of glassy graphite.
U.S. Applications 2008/0054106 and 2008/0220558 by Zehavi et al. demonstrate solutions to the obstacles from the deposition perspective. Chu, in U.S. Pat. No. 4,077,818, discusses deposition of silicon directly on graphite; the method consists of first applying silicon to raw graphite in order to create a surface that acts as a barrier and deposition interface for the active layers of silicon to be deposited. While this process might resolve issues of graphite impurity migration into a silicon solar cell, it still does not answer the issues of thermal expansion, CTE matching, and graphite integrity. Jones, in U.S. Pat. No. 3,078,328, discloses manufacture of solar cells in which a layer of silicon is grown onto a graphite layer from a silicon melt and doped to form a n-type layer; the melted silicon is contacted with a graphite substrate in order to generate a strong bond that includes the formation of silicon carbide (SiC) at the interface. While Jones attempts to answer the question of silicon adherence to graphite, the patent does disclose a feasible solution for the low cost manufacture of solar cells in terms of the complexity of the problem and the inherent problem of separation of graphite layers. The instant invention discloses novel methods to solve the obstacles inherent in the use of graphite substrates for solar cell manufacture.
Photovoltaic solar cells are semiconductor devices that convert sunlight into electricity. Much literature exists on the methods of manufacture and the performance of solar cells. NREL (National Renewable Energy Laboratory) of the US Dept of Energy frequently updates a chart of the best efficiencies achieved for photovoltaic devices in research labs. This chart is online at: nrel.gov/ncpv/thin_film/docs/kaz_best_research_cells.ppt; a version downloaded on Jul. 26, 2010 is FIG. 4.
From the published data, for single junction cells, single crystalline silicon is consistently the most efficient material for solar cells in terms of light to electricity conversion. For the purposes of mass production of solar cells, single crystal silicon is at a disadvantage in terms of cost. Thin film devices, while less efficient in the conversion of light into electricity, are much more cost effective for mass production.
Additional attempts have been documented in the literature, for example in Tamura, to demonstrate the feasibility of depositing a photoactive layer on inexpensive substrates to significantly reduce the cost of mass production of solar cells. Advances in the deposition techniques proposed by Tamura have been demonstrated by Zehavi et al. in U.S. 2008/0220558. In spite of the improvements found in the prior art methods, there remains a need for a barrier layer when working with a non-silicon substrate.
Dielectric barrier layers, with arrays of vias for use with conductive substrates have been disclosed in the literature. For example Barnett, U.S. Pat. No. 5,057,163 has described such a process. However, the process described did not account for diffusion of contaminants, or the potential junction created at the back plane through the barrier layer.
One innovation disclosed by the instant invention is the application of a highly conductive, doped layer, optionally silicon, between a conductive 102, optionally non-conductive 103, substrate and a substrate barrier layer 104. A substrate barrier layer improves the efficiency of the solar cell while still providing a path for photocurrent collection via a conductive substrate.